1. Field of the Invention
The present invention relates to an optical apparatus such as a flying height tester for determining a space between a first surface and a second surface, such as the space between a slider and a transparent rotating disk.
2. Background Information
Hard disk drives contain magnetic transducers which write and read information onto a rotating magnetic disk(s). The transducers are typically integrated into a slider that is assembled to a flexure arm. Some sliders contain a transducer to write information and a separate transducer to read information. The read transducer may be constructed from a magneto-resistive (MR) material. The slider and arm are commonly referred to as a head gimbal assembly (HGA). Each HGA is attached to an actuator arm that can move the sliders across the surfaces of the disk(s).
Each slider has an air bearing surface which cooperates with an air flow generated by the rotating disk(s) to create an air bearing between the disk and the transducer. The air bearing prevents mechanical wear between the slider and the disk surface. It is desirable to minimize the length of the space which separates the transducer and the disk to maximize the magnetic coupling between the two components. Sliders are therefore designed to create an optimal space between the transducer and the disk.
It is desirable to measure the thickness of an air bearing created by a slider. The thickness, or "flying height," is typically measured with an optical system that places a slider adjacent to a rotating transparent disk. A light beam is then directed through the transparent disk and reflected from the slider back to a photodetector. The detected light is used to compute the flying height of the slider. The current industry standard is the Dynamic Flying Height Tester (DFHT) sold by Phase Metrics, Inc. the assignee of the present application. The DFHT utilizes multiple wavelength intensity based interferometry to determine the flying height. The operation of the DFHT is discussed in U.S. Pat. No. 5,280,340 issued to Lacey, and U.S. Pat. No. 5,457,534 issued to Lacey et al.
The transducers may be fabricated by depositing thin films of material onto a wafer substrate constructed from AlTiC. A final capping layer of Al.sub.2 O.sub.3 is deposited to protect the transducer. The wafer is then sliced into individual slider elements such that the air bearing surface is primarily AlTiC material. The AlTiC air bearing surfaces may be further covered with a protective coating of diamond-like carbon (DLC).
The sliders may have rails located along an air bearing surface which assist in creating the air bearing. The transducers are located at the trailing edge of the rails. The sliders typically "fly" at an oblique angle relative to the disk so that the transducers are closer to the disk surface than most of the surface of the rails.
Flying height testers are typically operated so that the light beam is reflected from the rails of the slider. Unfortunately this does not provide an accurate measurement of the distance between the disk and the transducer located at the trailing edge of the slider. As discussed in an article by Yufeng Li, ASME/STLE Joint Tribology Conference, San Francisco, Calif., Oct. 13-17, 1996, it is desirable to direct the light beam onto the Al.sub.2 O.sub.3 cap of the transducer to evaluate the tribological and magnetic performance of the slider.
AlTiC is a granular material which may have varying grain sizes throughout the air bearing surface. The non-uniform grain sizes can vary the optical properties at different locations of the air bearing surface. To accurately determine the flying height, the DFHT must be calibrated to compensate for changes in optical properties. The DFHT is calibrated by moving the slider away from the disk and recording data.
The DFHT has a loader which places the slider adjacent to the disk. The loader pivots between a load position and an unload position. The pivotal movement of the slider during the calibration routine introduces a rotation which may change the location on the slider from which the light beam is being reflected. Unfortunately, the new location may have a different optical property that will degrade the accuracy of the calibration routine and the measurements by the flying height tester.
This problem becomes particularly acute when trying to measure the Al.sub.2 O.sub.3 cap. Al.sub.2 O.sub.3 has optical properties which are different than the optical properties of AlTiC. Therefore movement of the light beam from Al.sub.2 O.sub.3 to AlTiC during the calibration routine may create an inaccurate calibration and incorrect flying height measurements because of the drastic change in optical properties. It would therefore be desirable to provide a flying height tester which can accurately measure at the Al.sub.2 O.sub.3 cap of a hard disk drive slider.
The DFHT utilizes algorithms that are dependent upon the complex index of refraction (n and k), which is an optical property of the slider material. The complex index of refraction for each batch of sliders is typically measured with an ellipsometer and stored in the DFHT for use in computing the flying height. Having to separately determine the complex index of refraction is time consuming and thus increases the cost of testing the sliders and mass producing hard disk drives.
There has been marketed a flying height tester by Zygo Corp. under the trademark PEGASUS 2000 FHT which determines both the complex index of refraction and the flying height of a slider. The Zygo machine is further explained in U.S. Pat. No. 5,557,399 issued to DeGroot. The Zygo machine utilizes a polarized light beam that is reflected from the disk and the slider at an oblique angle. The approach implemented in the Zygo machine has the following problems.
It is difficult to analyze a polarized beam that is reflected from a spinning glass disk. The centrifugal forces generate stresses which make the glass birefringent. Birefringence is a characteristic of some materials that can be described as if the material had different indices of refraction depending on the polarization state of the light. In the context of Polarization-based FHTs, such as the Zygo machine, it means that the polarization states of both the probing beam and of the reflected beam that carries the information on flying height are changed as they travel through the glass disk. These effects are of such magnitude in regular flying height testing conditions that they render the measurements meaningless if they are not corrected. A complex correction method for the Zygo machine is described in U.S. Pat. No. 5,644,562, issued to De Groot.
The requirement for oblique incidence poses difficulties when it comes to accurately positioning the measurement spot on the surface of the slider. The Zygo machine requires a compromise between either determining the spot position from a low quality, high angle perspective, or necessitates an extra normal incidence view channel that must be precisely registered to the oblique measurement channel. This registration is susceptible to mechanical drift and in addition needs to be redone every time the transparent disk is changed, a task that is regularly performed in mass testing environments. Additionally, even though the three unknowns of flying height testing, flying height, and the components of the complex index of refraction n and k, are independent variables, they are closely coupled in the ellipsometric-type model that is used in the Zygo machine to interpret the data, resulting in solutions that are naturally unstable and tend to produce correlated fluctuations in the flying height and complex index of refraction.
Another problem in measuring flying heights is the granular nature of the AlTiC slider material. When a beam of light impinges on the slider surface, it is not cleanly reflected in a specular fashion. Instead, a large fraction of the light is scattered off in a diffuse manner, and even the portion of the light that reflects specularly gets significantly depolarized. It is extremely difficult to properly account for these effects in the theoretical model that is used for analyzing the data, since all the common approximations break down in the regime in which the grain size is of the order of the wavelength of the light. This results in having to introduce other ad hoc correction factors, such as what percentage of the light actually makes it back to the detector after reflection ,and what fraction of the detected light retains the polarization information required to determine the flying height.
The DLC coating of the air bearing surface introduces yet another problem. An error in the flying height measurement is introduced in the Zygo machine since the model interprets the various reflections from the different interfaces within the material as a single reflection, caused by an average material, located at a certain depth beneath the actual surface. This effect appears also at normal incidence, but for DLC-coated AlTiC its magnitude is close to a factor of 5 times smaller at normal incidence that at angles between 45 and 60 degrees used in the Zygo machine. The effect is important if the indices of refraction of the substrate (AlTiC in this case), and the coating (DLC) are very different.
It would be desirable to have a flying height tester which can accurately measure the distance between a disk and a slider while overcoming the noted problems in the prior art.